Method for measurement report event operation and network signaling in UE autonomous handover

ABSTRACT

The present disclosure relates to a communication technique for fusing, with an IoT technology, a 5G communication system for supporting a higher data transfer rate than a 4G system, and a system therefor. The present disclosure may be applied to intelligent services, such as smart homes, smart buildings, smart cities, smart cars or connected cars, health care, digital education, retailing, security and safety-related services, etc. on the basis of 5G communication technologies and IoT-related technologies. Disclosed in the disclosure is a method for measurement report/event operation and network signaling in an UE autonomous handover.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/610,880,filed Nov. 4, 2019, now U.S. Pat. No. 11,412,429, which is the 371National Stage of International Application No. PCT/KR2018/005208, filedMay 4, 2018, which claims priority to Korean Patent Application No.10-2017-0057093, filed May 4, 2017, and Korean Patent Application No.10-2018-0042177, filed Apr. 11, 2018, the disclosures of which areherein incorporated by reference in their entirety.

BACKGROUND 1. Field

Handover described in this patent is UE autonomous handover, andgenerally refers to handover in which a terminal receives handoverconfiguration signaling to measurement configuration signaling tohandover command signaling from a base station and accesses a targetbase station according to an autonomous determination based on ahandover execution condition indicated in the signaling. In addition,network handover refers to handover in which access to a target basestation is performed as indicated by a base station when a handovercommand is received from the base station as in the existing LTE.

2. Description of Related Art

In order to satisfy wireless data traffic demands that tend to increaseafter the 4^(th)-generation (4G) communication system commercialization,efforts to develop an enhanced 5G communication system or a pre-5Gcommunication system are being made. For this reason, the 5Gcommunication system or pre-5G communication system is called a beyond4G network communication system or a post LTE system. In order toachieve a high data transfer rate, the 5G communication system isconsidered to be implemented in a mmWave band (e.g., 60 GHz band). Inorder to reduce a loss of electric waves and increase the transferdistance of electric waves in the mmWave band, beamforming, massiveMIMO, full dimensional MIMO (FD-MIMO), array antenna, analogbeam-forming and large scale antenna technologies are being discussed inthe 5G communication system. Furthermore, in order to improve thenetwork of a system, technologies, such as an improved small cell, anadvanced small cell, a cloud radio access network (cloud RAN), anultra-dense network, device to device communication (D2D), wirelessbackhaul, a moving network, cooperative communication, coordinatedmulti-points (CoMP) and reception interference cancellation, are beingdeveloped in the 5G communication system. In addition, hybrid FSK andQAM modulation (FQAM) and sliding window superposition coding (SWSC)that are advanced coding modulation (ACM) schemes, improved filter bankmulti-carrier (FBMC), non-quadrature multiple access (NOMA) and sparsecode multiple access (SCMA) are being developed in the 5G system.

The Internet evolves from a human-centered connection network over whichhuman generates and consumes information to Internet of Things (IoT) inwhich information is exchanged and process between distributed elements,such as things. An Internet of Everything (IoE) technology in which abig data processing technology through a connection with a cloud serveris combined with the IoT technology is emerging. In order to implementthe IoT, technical elements, such as the sensing technology,wired/wireless communication and network infrastructure, serviceinterface technology and security technology, are required. Accordingly,technologies, such as a sensor network, machine to machine (M2M) andmachine type communication (MTC) for a connection between things, arerecently researched. In the IoT environment, an intelligent Internettechnology (IT) service in which a new value is created for human lifeby collecting and analyzing data generated from connected things may beprovided. The IoT may be applied to fields, such as a smart home, asmart building, a smart city, a smart car or a connected car, a smartgrid, health care, smart home appliances, and advanced medical services,through convergence and composition between the existing informationtechnology (IT) and various industries.

Accordingly, various attempts to apply the 5G communication system tothe IoT are being made. For example, 5G communication technologies, suchas a sensor network, machine to machine (M2M) and machine typecommunication (MTC), are implemented by schemes, such as beamforming,MIMO, and an array antenna. The application of a cloud wireless accessnetwork (cloud RAN) as the aforementioned big data processing technologymay be said to be an example of convergence between the 5G technologyand the IoT technology.

With the recent development of long term evolution (LTE) andLTE-Advanced, there is a need for a method and apparatus for improving ameasurement report/event operation and network signaling in UEautonomous handover.

There is proposed a method of improving a measurement report/eventoperation and network signaling in UE autonomous handover.

SUMMARY

According to the disclosure, a communication method of a terminal mayinclude transmitting, to a serving base station, a first messageincluding information on a candidate target cell, receiving, from theserving base station, a second message including information on acondition for conditional handover related to the candidate target cell,and performing handover to the candidate target cell if a measurementresult for the candidate target cell meets the condition for theconditional handover.

Furthermore, the information on the condition for the conditionalhandover may include at least one of ID information of the candidatetarget cell, measurement ID information, delta information to which ameasurement configuration based on the measurement ID information is tobe applied, or access stratum (AS) configuration information to be usedif handover to the candidate target cell has been performed.

Furthermore, the method may further include transmitting, to the servingbase station, a third message to request the candidate target cell to beremoved from a subject of the conditional handover based on themeasurement result for the candidate target cell.

Furthermore, in the method, the first message may be transmitted if thecurrently set number of candidate target cells is smaller than thepreset maximum number of candidate target cells.

Furthermore, a communication method of a base station may includereceiving, from a terminal, a first message including information on acandidate target cell, transmitting, to the candidate target cell, asecond message including configuration request information forconditional handover, receiving, from the candidate target cell, a thirdmessage including configuration approval information for the conditionalhandover, configuring a condition for the conditional handover in whichthe terminal needs to perform handover to the candidate target cell, andtransmitting, to the terminal, a fourth message including information onthe condition for the conditional handover related to the candidatetarget cell.

Furthermore, the method may further include receiving, from theterminal, a fifth message to request the candidate target cell,determined based on a measurement result for the candidate target cell,to be excluded from a subject of the conditional handover.

Furthermore, receiving the first message may include transmittinginformation on a preset maximum number of candidate target cells to theterminal and receiving the first message if the currently set number ofcandidate target cells is smaller than the preset maximum number ofcandidate target cells.

Furthermore, a terminal may include a transceiver and a controllerconfigured to transmit, to a serving base station, a first messageincluding information on a candidate target cell, receive, from theserving base station, a second message including information on acondition for conditional handover related to the candidate target cell,and perform handover to the candidate target cell if a measurementresult for the candidate target cell meets the condition for theconditional handover.

Furthermore, a base station may include a transceiver and a controllerconfigured to receive, from a terminal, a first message includinginformation on a candidate target cell, transmit, to the candidatetarget cell, a second message including configuration requestinformation for conditional handover, receive, from the candidate targetcell, a third message including configuration approval information forthe conditional handover, configure a condition for the conditionalhandover in which the terminal needs to perform handover to thecandidate target cell, and transmit, to the terminal, a fourth messageincluding information on the condition for the conditional handoverrelated to the candidate target cell.

According to an embodiment of the disclosure, a measurement report/eventoperation and network signaling in UE autonomous handover can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a measurement report/event operationaccording to an embodiment of the disclosure.

FIG. 2 is a diagram illustrating a measurement report/event operationaccording to another embodiment of the disclosure.

FIG. 3 is a diagram illustrating mobilityControlInfo according to anembodiment of the disclosure.

FIG. 4 is a diagram illustrating a handover operation according to anembodiment of the disclosure.

FIG. 5 is a diagram illustrating a handover operation according toanother embodiment of the disclosure.

FIG. 6 is a diagram illustrating an embodiment of a case where a celladdition MR and a cell leaving MR overlap.

FIG. 7 is a diagram illustrating an embodiment in which a cell leavingMR is processed after cell addition MR transmission.

FIG. 8 is a diagram illustrating another embodiment in which a cellleaving MR is processed after cell addition MR transmission.

FIG. 9 is a diagram illustrating yet another embodiment in which a cellleaving MR is processed after cell addition MR transmission.

FIG. 10 is a diagram illustrating yet another embodiment in which a cellleaving MR is processed after cell addition MR transmission.

FIG. 11 is a diagram illustrating an operation of processing a candidatecell and MR configuration according to an embodiment of the disclosure.

FIG. 12 is a diagram illustrating an operation of processing a candidatecell and MR configuration according to another embodiment of thedisclosure.

FIG. 13 is a diagram illustrating an operation of processing a candidatecell and MR configuration according to yet another embodiment of thedisclosure.

FIG. 14 is a diagram illustrating an operation for a target cell tonotify candidate cells of a state change after a terminal performs HOaccording to an embodiment of the disclosure.

FIG. 15 is a diagram illustrating an operation for a terminal to switchto a candidate cell when an RLF occurs according to an embodiment of thedisclosure.

FIG. 16 is a diagram illustrating the configuration of a terminalaccording to an embodiment of the disclosure.

FIG. 17 is a diagram illustrating the configuration of a base stationaccording to an embodiment of the disclosure.

FIG. 18 is a diagram illustrating an operation method of conditional HOaccording to an embodiment of the disclosure.

FIG. 19 is a diagram illustrating an operation method of conditional HOaccording to another embodiment of the disclosure.

FIG. 20 is a diagram illustrating an operation method of conditional HOaccording to yet another embodiment of the disclosure.

FIG. 21 is a diagram illustrating an operation method of conditional HOaccording to yet another embodiment of the disclosure.

DETAILED DESCRIPTION

Hereinafter, embodiments of the disclosure are described in detail withreference to the accompanying drawings. Furthermore, in describing thedisclosure, a detailed description of a related known function orconfiguration will be omitted if it is deemed to make the gist of thedisclosure unnecessarily vague. Furthermore, terms to be describedhereunder have been defined by taking into consideration functions inthe disclosure, and may be different depending on a user, an operator'sintention or practice.

The merits and characteristics of the disclosure and a method ofachieving the merits and characteristics will become more apparent fromthe embodiments described in detail in conjunction with the accompanyingdrawings. However, the disclosure is not limited to the disclosedembodiments, but may be implemented in various different ways. Theembodiments are provided to only complete the disclosure and to allowthose skilled in the art to fully understand the category of thedisclosure. The disclosure is defined by the category of the claims. Thesame reference numerals will be used to refer to the same or similarelements throughout the drawings.

<Embodiment of M-Based Candidate Cell Operation>

FIG. 1 is a diagram illustrating a measurement report/event operationaccording to an embodiment of the disclosure.

A base station 120 sets an M value for a given event and notifies aterminal 110 of the set M value through dedicated signaling (Measurementconfig) (step 140). The given event may correspond to an event that theterminal reports a candidate target cell. M is a maximum m value ofcandidate target cell managed by the terminal 110 or the base station120. In this case, a configuration for UE autonomous handover may beindicated in Measurement config configured by the base station 120. TheUE autonomous handover configuration may be different from a networkhandover configuration. The terminal 110 performs an operation ofproducing a current candidate cell list for the maximum m value M withrespect to the corresponding measurement event based on the indication,storing the current candidate cell list, increasing a variable m valueby 1 when an entering event is triggered, and decreasing the m value by1 when a leaving event is triggered (step 150). In this case, the givenevent may be an event corresponding to a case where a receivedconnection quality value of the target cell is great by a given value, areceived connection quality value of the target cell is greater than areceived connection quality value of a serving cell by a given value, areceived connection quality value of the serving cell is small by agiven value, or a received connection quality value of the serving cellis a given value or less and a received connection quality value of thetarget cell is another given value or more. Furthermore, the given eventmay be an event corresponding to a case where a given CSI-RS signal ofthe target cell is a given threshold value or more or a given CSI-RSsignal of the serving cell is a given threshold value or less, a givenCSI-RS signal of the serving cell is a given value or less and a givenCSI-RS signal of the target cell is another given value or more, or agiven CSI-RS signal of the target cell is greater than a received signalvalue of a given CSI-RS of the serving cell by a given threshold value.Furthermore, the given event may be an event corresponding to a casewhere a received connection quality value of an inter-RAT target cell isa given threshold value or more, a received connection quality value ofa Pcell is a given threshold value or less, or a received connectionquality value of an inter-RAT target cell is another given thresholdvalue or more.

In this case, a reference signal for measuring the connection quality ofthe cell is defined in a measurement configuration, and may be a synchsignal, a cell specific reference signal (CRS), or a channel stateinformation reference signal (CSI-RS).

If a value of m does not exceed M when the given event occurs, theterminal 110 may transmit a measurement report (MR) signal, includingcandidate target cell information satisfying the given event, to thebase station 120 (steps 160 and 170). Specifically, when thecorresponding event is triggered, the terminal 110 stores the cell ID ofan event target cell (this means a neighbor cell in A3 or A4, a neighborcell having a value greater than a threshold value in A5, a neighborcell having a value greater than a threshold value of an Scell in A6, acell that has transmitted a CSI-RS in which an event has occurred in C1or C2, or a neighbor cell having a value better than a threshold valuein B1 or B2) 130, and transmits the measurement report (MR) to the basestation 120. The MR signal may include at least one of the ID of acandidate target cell that meets the given event, the beam index of acandidate target cell 130, the received signal intensity of thecandidate target cell, the ID of another surrounding cell, the beamindex of a surrounding cell, or the received signal intensity of thesurrounding cell. The base station 120 that has received the MR signalmay identify whether the candidate target cell may become the candidatetarget cell 130 for the terminal 110 by performing a negotiationprocedure with the candidate target cell 130 (step 180). A HOrequest/ack signal may be used in the negotiation procedure between thebase stations.

FIG. 2 is a diagram illustrating a measurement report/event operationaccording to another embodiment of the disclosure.

A serving base station 120 may transmit, to a terminal 110, ameasurement configuration signal including at least one of M indicativeof a maximum number of candidate target cells, an event in which a UEautonomous handover usage measurement report will be performed, anentering condition for the event, or a leaving condition for the event(step 210).

If an entering condition for the corresponding event is met (step 213,step 230) when the terminal 110 makes a measurement report, the terminalmay make a measurement report (step 215, step 233). If the terminal 110receives, from the serving base station 110, a confirmation signal sothat a cell ID meeting the entering condition is included in a candidatetarget cell list, the terminal increases an m value by 1 (steps 217,219, 220, 223, 225, 227, 245, 250, 255, 257, 259). If the m has reachedan M value, the terminal does not make a measurement report for any cellthat initiates a corresponding event.

If the leaving condition for the corresponding set event is met (step237), the terminal 110 transmits a measurement report for thecorresponding event to the serving base station 120 again (step 240). Ifthe terminal 110 receives, from the serving base station 120,confirmation signaling by which the ID of a cell meeting the event isdeleted from the candidate target cell list, the terminal deletes the IDof the cell meeting the leaving condition and decreases the m value by 1(steps 243, 247, 253, 257, 259).

If the m value is equal to the M, the terminal 110 omits a report ordoes not make a report for a cell newly triggered by a correspondingevent.

If there is a cell on which the terminal 110 has not performed MRtransmission although m reaches M and an event is met, thereafter, ifthe m value is reduced and smaller than the M at any timing (the m valuebecomes smaller than M if a measurement report is transmitted and aconfirmation signal is received with respect to a cell meeting a leavingcondition) or if the m value may become smaller than M (when ameasurement report is transmitted with respect to a cell meeting aleaving condition), if a corresponding cell still meets a condition inwhich an event is triggered at current timing (the state in which anentering condition has been met before a time to trigger time based on acurrent time) (step 260), the terminal 110 may transmit, to the servingbase station 120, an MR including corresponding cell information (a cellID, the received signal intensity of the cell) (steps 263, 265, 267,270, 273, 275, 277).

For reference, the terminal 110 increases the m value with reference totarget cell addition information (cell ID, indicates that a target cellhas been added) included in the confirmation signal, and decreases the mvalue based on cell release or information (cell ID, indicates that atarget cell has been removed) indicating that a target cell has beenremoved. If a cell ID is included in a confirmation signal withoutindication for a target cell added or removed, the terminal 110 mayincrease the m value when receiving a cell ID that has transmitted an MRfor addition through a confirmation signal, and may decrease the m valuewhen transmitting an MR for a cell to be removed (meeting a leavingcondition).

In FIG. 2 , the confirmation signal may correspond to an RRC connectionreconfiguration in step 225 or 257 or 275. The terminal 110 may transmitan ack signal for the confirmation signal in step 227 or 259 to 277.

The serving base station 120 may indicate a handover event which may beapplied when the terminal 110 performs UE autonomous handover on one ofcandidate target cells through signaling in step 210 or 225 or 257 or275. If a target cell meeting the handover event indicated through thesignaling of the serving base station 120 in step 210 or 225 or 257 or275 is discovered among the candidate target cells, the terminal 110 mayperform handover to the target cell (step 280). The access procedure forthe target cell 132 corresponds to a step after the synchronization step(step 283) of FIG. 2 b.

FIG. 3 is a diagram illustrating mobilityControlInfo according to anembodiment of the disclosure.

A mobilityControlInfo IE may be included in the signaling correspondingto step 210 or 225 or 257 or 275 of FIG. 2 . The mobilityControlInfo IEis used to configure at least one of a measurement configuration for UEautonomous handover or NW handover to a handover configuration necessaryto perform handover on one of candidate target cells to a handoverconfiguration for a target cell. Specifically, the mobilityControlInfoIE first includes a factor to identify whether corresponding informationis based on network handover or UE autonomous handover. If thecorresponding information is a UE autonomous-based factor, it indicateswhether the UE autonomous-based factor is a target cell addition or atarget cell release, and may indicate a target cell ID. ThemobilityControlInfo IE may include a condition for UE HO execution alongwith the addition and release indication. The condition for UE HOexecution may be the same as the event for MR transmission described inFIG. 1 . One or more handover execution conditions may be included. Ifthe number of handover execution conditions is plural, the plurality ofhandover execution conditions may be used at the same time. If ahandover execution event is newly transmitted by the serving basestation 120, the terminal 110 may overwrite a previously received UE HOexecution condition. In a next structure, in the case of a target celladdition, the mobilityControlInfo IE may transmit resource configurationinformation in a target cell which is necessary for the terminal 110 toperform handover. If the terminal 110 is configured with network-basedHO based on the mobilityControlInfo IE, the terminal performsnetwork-based HO based on given AS configuration information. If theterminal is configured with UE-based HO, the terminal determines whethera target cell is added or released. If the target cell is to be added,the terminal stores resource configuration information for thecorresponding add target cell. The existing stored configurationinformation of the cell may be overwritten with new configurationinformation if the terminal 110 subsequently receives the newconfiguration information as target cell addition of UE-based HO througha mobilityControlInfo IE. In this case, the terminal 110 may also storeall types of current AS configuration information of the terminal. IfUE-based HO is performed and a corresponding target cell is a cell thathas previously stored AS configuration information, the terminal 110 mayaccess the target cell based on the stored configuration information anduse the information for a connection. A MobilityControlInfo IE-basedmeasurement configuration to a handover configuration-related operationmay be used for an M update without any change.

FIG. 4 is a diagram illustrating a handover operation according to anembodiment of the disclosure.

A terminal 110 transmits an MR based on an event indicated by a servingbase station 120. The serving base station 120 transmits an HO requestto an adjacent base station 130 that may become a candidate target cell(step 410, step 415). If the corresponding adjacent base station 130 isidentified to be capable of becoming a candidate target cell, ittransmits ACK to the serving base station (step 420). The adjacent basestation 130 may provide AS configuration information which may be usedfor the terminal 110 to access the corresponding base station 130 alongwith the ACK. The AS configuration information may be delta informationor full AS configuration information of UE AS context in a Time 1. Theserving base station 120 instructs the terminal 110 to add the adjacentbase station 130 as a candidate target cell and also transmits the ASconfiguration information, received from the adjacent base station 130,to the terminal 110 (step 425). The terminal 110 transmits areconfiguration complete signal to the serving base station 120, and maystore the received AS configuration of the adjacent base station 130(step 430, step 435). That is, in the state in which the candidatetarget cell and the configuration information in the candidate targetcell are maintained in the terminal 110 and UE autonomous HO has not yetbeen performed, if an RRC connection reconfiguration not handoverindication is transmitted by the serving base station 120, the terminal110 is aware that the RRC connection reconfiguration is a configurationfor the serving cell and performs the AS configuration based oncorresponding reconfiguration information.

Thereafter, the serving base station 120 may transmit an RRC connectionreconfiguration (step 440). The RRC connection reconfiguration mayinclude AS configuration change information in the serving cell. Theterminal 110 transmits a reconfiguration complete signal to the servingbase station 120, and may reconfigure the AS context of the serving cellbased on the received AS configuration (step 445, step 450). Thereafter,if a handover event for the T-gNB 130 occurs, the terminal 110 mayperform a connection configuration for the corresponding target basestation 130 using the AS configuration information of the T-gNB storedin the Time1 (step 455). The terminal 110 may perform a random accessprocedure with the T-gNB 130 (step 460). If the connection configurationprocedure is completed, the terminal 110 may transmit a HO completesignal to the T-gNB 130 (step 465). The T-gNB 130 that has received theHO complete signal may notify the serving base station 120 of the HOcomplete of the terminal 110 (step 470).

In the case of an AS configuration for UE autonomous HO, the terminal110 stores all types of AS configuration information of the terminal 110at timing when a corresponding configuration is performed in addition tothe configuration (full configuration or delta configuration) performedby a target cell through a HO command. Accordingly, when UE autonomousHO is performed, the terminal 110 accesses a corresponding target cellbased on AS configuration information stored in each target cell.

FIG. 5 is a diagram illustrating a handover operation according toanother embodiment of the disclosure.

An M value is set as a maximum number at which a terminal 110 transmitsan MR because an event for target cell addition is triggered. A servingbase station 120 indicates, with respect to the terminal 110 at leastone of an M value, an event for UE autonomous HO, a candidate targetcell addition condition or leaving condition, or an event for NW basedHO through measurement configuration signaling (step 510). The terminal110 may identify whether an adjacent base station meeting a candidatetarget cell addition condition is discovered based on the event for UEautonomous HO indicated in the measurement configuration by performingreceived signal intensity measurement for the serving cell and aneighbor cell. If an adjacent base station 130 meeting the candidatetarget cell addition condition is discovered and them value is notgreater than an M value, the terminal 110 increases the m value by 1.The terminal 110 transmits, to the serving base station 120, an MRincluding information on the adjacent base station 130 meeting thecandidate target cell addition condition (step 513, step 515). The MRincludes at least one of the cell ID of the adjacent base station 130,the beam index of the adjacent base station 130, the received signalintensity of the adjacent base station 130, the cell ID of a differentadjacent base station, the beam index of the different adjacent basestation, or the received signal intensity of the different adjacent basestation. The serving base station 120 and the adjacent base station 130may identify a candidate target cell of the terminal 110 by exchangingan HO request and an HO request ack based on the MR of the terminal 110(steps 517, 520, 523, 525). The serving base station 120 performs an RRCconnection reconfiguration/RRC connection reconfiguration completeprocedure in order to provide identified candidate target cellinformation to the terminal 110 (step 527, step 530). Thereafter, if anew neighbor cell 131 meeting the candidate target cell additioncondition is identified to be present based on a configured event, theterminal 110 identifies whether the m value is not greater than the Mvalue, and may transmit an MR to the serving base station 120 if the mvalue is not greater than the M value (step 533, step 535). The MRincludes at least one of the cell ID of the new adjacent base station131, the beam index of the new adjacent base station 131, the receivedsignal intensity of the new adjacent base station 131, the cell ID of adifferent adjacent base station, the beam index of the differentadjacent base station, or the received signal intensity of the differentadjacent base station. The serving base station 120 and the adjacentbase station 131 may identify a candidate target cell of the terminal110 by exchanging an HO request and an HO request ack based on the MR ofthe terminal 110 (steps 537, 540, 543, 545). The serving base station120 performs an RRC connection reconfiguration/RRC connectionreconfiguration complete procedure in order to provide determinedcandidate target cell information to the terminal 110 (step 547, step550). If a new neighbor cell 132 meeting the candidate target celladdition condition is identified to be present based on a configuredevent, when the m value is greater than the M value, the terminal 110does not perform MR transmission although the target cell additioncondition is met (step 553).

If the leaving condition of an event configured for a cell ID includedin a candidate target cell list is met, the terminal 110 transmits an MRto the serving base station 120. The MR includes at least one of the IDof an neighbor cell meeting the candidate target cell leaving condition,the received signal intensity of the neighbor cell, the beam index ofthe neighbor cell, the ID of a different neighbor cell, the beam indexof the different neighbor cell, or the received signal intensity of thedifferent neighbor cell. The serving base station 120 performs acandidate cell removal procedure with the corresponding cell meeting thecandidate target cell leaving condition, and transmits resultinformation to the terminal 110. That is, the result informationincludes a target cell ID for which candidate cell removal is indicatedalong with UE autonomous handover indication. If the terminal 110already stores the target cell ID as a current candidate cell based onthe indication, it removes the ID and stored configuration informationfrom the candidate cell list. In another embodiment, if one cell isremoved from the candidate target cell, the terminal 110 may decreasethe m value by 1. If a UE autonomous handover event execution conditionis met with respect to the candidate target cell (step 555), theterminal 110 may identify the corresponding cell as the target basestation 130 and perform a connection reconfiguration procedure on thecell. The connection reconfiguration procedure for the target basestation 130 corresponds to a step after a synchronization procedure(steps 557, 560, 563, 565, 567, 570, 573, 575).

<In Another Embodiment, a Serving Base Station Manages a CandidateTarget Cell for UE Autonomous HO by Managing an M Value and Adjusting aValue for an Event>

A serving base station 120 may indicate a candidate target cellconfiguration event, a candidate target cell addition condition and acandidate target cell leaving condition with respect to a terminal 110.The serving base station 120 manages a maximum number of candidatetarget cells that may be managed with respect to the terminal 110, thatis, an M value. That is, the terminal 110 does not need to manage the Mvalue, and only the serving base station 120 manages the M value. Theterminal 110 transmits an MR according to an event indicated by theserving base station 120. In a process for the serving base station 120to identify a candidate target cell based on the MR of the terminal 110,the serving base station 120 identifies whether the number of candidatetarget cells for the terminal 110 exceeds the M value. If the number ofcandidate target cells is greater than the M value, the serving basestation 120 may indicate the re-adjustment of an addition condition oran event re-adjusted with respect to the terminal 110 so that theterminal 110 reduces candidate target cell addition. If the number ofcandidate target cells is greater than the M value, the serving basestation 120 may indicate the re-adjustment of a leaving condition or anevent re-adjusted with respect to the terminal 110 so that the terminal110 increases candidate target cell leaving. That is, the serving basestation 120 adjusts the parameter (factor) of a candidate target celladdition/deletion event and indicates a new event configuration for theterminal 110 so that a candidate cell corresponding to the M value ismaintained.

Embodiments of various options for adjusting the event are as follows.

Option 1. When the number of candidate cells is M, a value of an eventis adjusted in order to maintain the M value and a new event1configuration is indicated in the terminal 110. For example, if Mcandidate cells are maintained, the serving base station 120 raises athreshold value of an event in which an MR is transmitted, for example,a threshold value in the case of A4, configures the threshold value, andtransmits it to the terminal 110.

Option 2. When the number of candidate cells becomes 1, a value of anevent is adjusted in order to maintain an M value, and a new event1configuration is indicated in the terminal 110. For example, when thenumber of candidate cells becomes 1, the serving base station 120decreases a threshold value of an event in which an MR is transmitted,for example, a threshold value in the case of A4, and configures thethreshold value in the terminal 110. Accordingly, the possibility thatthe terminal 110 will perform an operation of transmitting an additionMR is increased.

Option 3. When the number of candidate cells becomes M, the serving basestation 120 may instruct the terminal 110 to not perform additionaltransmission of an MR corresponding to a corresponding event.

<An Embodiment in which a Serving Base Station May Perform Cell AdditionBased on the Cell ID of a Different Neighbor Cell and Cell QualityInformation of the Different Neighbor Cell Reported by a Terminal if aLeaving Condition for a Candidate Target Cell is Met is Described Below>

If an event meeting a leaving condition occurs among neighbor cellsincluded in a current candidate target cell list, a terminal 110transmits an MR to the serving base station 120. In this case, theterminal may transmit the MR, including the cell ID of a cell meeting apre-defined candidate target cell addition condition, among neighborcells, and a cell quality value, to the serving base station 120. Theserving base station 120 may identify whether a neighbor cell notincluded in the current candidate target cell list meets the celladdition condition based on the MR. If a neighbor cell meeting the celladdition condition is present, the serving base station 120 performs anegotiation for candidate cell addition on the corresponding cell, andmay instruct the terminal 110 to add the candidate cell as a candidatetarget cell.

In another embodiment, when cell addition is performed based on an MRtransmitted when a candidate target cell leaving condition occurs,reliability in adding a corresponding neighbor cell to a candidatetarget cell may be degraded because the neighbor cell may not meet atime to trigger (TTT) for meeting a cell addition event. Accordingly,the terminal 110 may transmit an MR along with each cell ID, cellquality and an indicator notifying whether TTT is met when informationof another neighbor cell is included in the MR that reports a cell IDmeeting a leaving condition.

The serving base station 120 may perform a negotiation for adding acell, meeting both a cell addition condition and a TTT condition, as acandidate cell. The base station 120 may indicate that the cell shouldbe added to a candidate target cell list by transmitting the added cellinformation to the terminal 110.

<An Embodiment of a Case where a Cell Addition MR and a Cell Leaving MROverlap>

FIG. 6 is a diagram illustrating an embodiment of a case where a celladdition MR and a cell leaving MR overlap.

A terminal 110 may be in the state in which the terminal has notreceived a HO command (or RRC reconfiguration with mobilityControlInfo)from a serving base station 120 (step 620) after transmitting a celladdition MR to the serving base station 120 (step 610). Furthermore, inthe state in which the terminal 110 has not received the Ho command, itmay measure the channels of surrounding base station and transmit a cellleaving MR to the serving base station 120 because a condition forleaving is met among surrounding base stations (step 630). In such acase, the serving base station 120 may receive a cell adding MR and acell leaving MR from the terminal 110 before it transmits a handovercommand. The serving base station 120 may identify a candidate cell tobe finally used by the terminal 110 based on base station informationincluded in the cell adding MR and cell leaving MR (step 640). In such acase, the serving base station 120 does not transmit a handover commandwith respect to each of the cell adding MR and the cell leaving MR, andmay transmit the final candidate cell information by transmitting asingle handover command to the terminal 110 (steps 650, 660, 670).Furthermore, after receiving the handover command, the terminal 110 maytransmit RRC Reconfiguration Complete to the serving base station 120(steps 680, 690).

FIG. 7 is a diagram illustrating an embodiment in which a cell leavingMR is processed after cell addition MR transmission.

After a terminal 110 measures the channel of a surrounding base station120, it may transmit a cell addition MR to the serving base station 120after a cell addition MR transmission condition is met (step 710). Afterthe cell addition MR transmission to the serving base station 120, theterminal 110 may be in the state in which it has not received a HOcommand (e.g., an RRC reconfiguration with mobilityControlInfo) (step720). Furthermore, the terminal 110 may need to transmit a cell leavingMR to the serving base station 120 because a cell leaving MRtransmission condition is met after the terminal 110 measures thechannels of surrounding base stations in the state in which the terminalhas not receive a HO Command (step 730). In such a case, the terminal110 may defer the cell leaving MR until it transmits RRC reconfigurationcomplete for a cell adding MR to the serving base station 120 (step760). For example, after the terminal 110 transmits a cell addition MRto the serving base station 120 and transmits RRC reconfigurationcomplete corresponding to the cell addition MR (step 740, 750), theterminal 110 may transmit a cell leaving MR to the serving base station120 (step 770).

FIG. 8 is a diagram illustrating another embodiment in which a cellleaving MR is processed after cell addition MR transmission.

After a terminal 110 identifies whether a condition for the cell addingMR transmission of a surrounding base station to a serving base station120 is met, the terminal may transmit a cell adding MR (step 810). Afterthe terminal 110 transmits the cell adding MR to the serving basestation 120, the terminal 110 may receive HO command incl. cell add.(e.g., RRC reconfiguration with mobilityControlInfo) from the servingbase station 120 (steps 820, 830). After the terminal 110 receives theHO command, the terminal may be in the state in which it has nottransmitted an RRC Reconfiguration complete message to the serving basestation 120. Furthermore, in the state in which the terminal 110 has nottransmitted the RRC Reconfiguration complete message, the terminal mayhas to measure the channel of a surrounding base station, identifywhether a condition for cell leaving MR transmission is met, and thentransmit a cell leaving MR (step 840). In such a case, the terminal 110may defer the cell leaving MR until it transmits RRC reconfigurationcomplete related to the cell adding MR (step 860). For example, afterthe terminal 110 transmits a cell adding MR to the serving base station120 and RRC reconfiguration complete corresponding to the cell adding MRis performed (step 850), the terminal 110 may transmit the cell leavingMR to the serving base station 120 (step 870).

FIG. 9 is a diagram illustrating yet another embodiment in which a cellleaving MR is processed after cell addition MR transmission.

A terminal 110 may identify whether a cell leaving MR transmissioncondition for a surrounding base station to a serving base station 120is met. If the condition is met, the terminal 110 may transmit a cellleaving MR (step 910, 920). The terminal 110 may be in the state inwhich it has not received a HO command (e.g., RRC reconfiguration withmobilityControlInfo) from the serving base station 120 aftertransmitting the cell leaving MR (step 930). Furthermore, in the statein which the terminal 110 has not received the HO command from theserving base station 120, the terminal 110 may have to transmit a celladdition MR to the serving base station 120 because a cell addition MRtransmission condition is met after the terminal measures the channelsof surrounding base stations (step 940). In such a case, the terminal110 may defer a cell addition MR operation (step 970) until it transmitsRRC reconfiguration complete for a cell leaving MR to the serving basestation 120 (steps 950, 960). For example, after the terminal 110transmits a cell leaving MR to the serving base station 120 andtransmits RRC reconfiguration complete corresponding to the cell leavingMR (step 960), the terminal 110 may transmit the cell addition MR to theserving base station 110 (step 980).

FIG. 10 is a diagram illustrating yet another embodiment in which a cellleaving MR is processed after cell addition MR transmission.

A terminal 110 may identify whether the cell leaving MR transmissioncondition of a surrounding base station to a serving base station 120 ismet, and may transmit a cell leaving MR if the condition is met (steps1010, 1020). After transmitting the cell leaving MR, the terminal 110may receive an HO command (e.g., RRC reconfiguration withmobilityControlInfo) from the serving base station 120 (steps 1030,1040). Furthermore, after receiving the HO command, the terminal 110 maybe in the state in which it has not transmitted RRC reconfigurationcomplete. In the state in which the terminal 110 has not transmitted theRRC reconfiguration complete, the terminal 110 may have to transmit acell addition MR to the serving base station 120 because a cell additionMR transmission condition is met after the terminal measures thechannels of surrounding base stations (step 1050). In such a case, theterminal 110 may defer a cell addition MR operation until it transmitsRRC reconfiguration complete for the cell leaving MR to the serving basestation 120 (step 1070). For example, after the terminal 110 transmitsthe cell leaving MR to the serving base station 120 and transmits RRCreconfiguration complete corresponding to the cell leaving MR (step1060), the terminal 110 may transmit the cell addition MR to the servingbase station 120 (step 1080).

<An Embodiment of an Operation for a Terminal to Process a CandidateCell and MR Configuration Used Upon Communication with a Serving Cellafter Performing Handover>

FIG. 11 is a diagram illustrating an operation of processing a candidatecell and MR configuration according to an embodiment of the disclosure.

<Option 1. An Operation for a Terminal 110 to Reset a Candidate Cell andMR Configuration Maintained Upon Communication with a Serving Cell 120after Performing Handover to a Target Cell 130>

For example, if the terminal 110 has successfully performed handover toone of selected candidate cells 130 through the method (steps 1110,1120, 1130, 1140, 1150, 1160, 1170), the terminal 110 may reset theselected candidate cell 130 and an MR configuration for the serving basestation through the method. Thereafter, the terminal 110 may perform anoperation of obtaining new MR configuration information from the movedserving base station 130 or selecting a new candidate cell (step 1180).

FIG. 12 is a diagram illustrating an operation of processing a candidatecell and MR configuration according to another embodiment of thedisclosure.

<Option 2. An Operation for a Terminal 110 to Maintain a Candidate CellReset and MR Configuration Maintained Upon Communication with a ServingCell 120 after Performing Handover to a Target Cell 130, and theTerminal Uses the Candidate Cell Reset and MR Configuration Until a NewMR Configuration is Transmitted by the Target Cell 130>

For example, if the terminal 110 has successfully performed handover toone of selected candidate cells 130 through the method (steps 1210,1215, 1220, 1225, 1230, 1235), the terminal 110 may reset only theselected candidate cell through the method (step 1250). Thereafter, theterminal 110 may perform an operation of selecting a new candidate cellfrom the base station 130 to which the terminal has moved (steps 1240,1245). Furthermore, the terminal 110 may maintain and use an existing MRconfiguration, and may use the existing MR configuration (step 1255)until it receives a new MR configuration from the base station 130 towhich the terminal has moved (step 1260).

FIG. 13 is a diagram illustrating an operation of processing a candidatecell and MR configuration according to yet another embodiment of thedisclosure.

<Option 3. A Terminal 110 Maintains a Candidate Cell and MRConfiguration Maintained Upon Communication with a Serving Cell 120after Performing Handover to a Target Cell 130. If a New MRConfiguration is Transmitted by the Target Cell 130, the TerminalUpdates the Candidate Cell Based on the New MR Configuration>

For example, if the terminal 110 has successfully performed handover toone of selected candidate cells 130 through the method (steps 1310,1315, 1320, 1325, 1330, 1335), the terminal 110 may maintain theselected candidate cell 130 and MR configuration method through themethod without resetting the selected candidate cell 130 and MRconfiguration (steps 1350, 1340, 1345). The terminal 110 may usecandidate cell information and an MR configuration previously used untilit receives a new MR configuration from the base station 130 to whichthe terminal has moved through the handover (steps 1355, 1360).

<A Target Cell Needs to Perform Signaling that Notifies Candidate Cellsof a State Change after a Terminal Executes HO>

FIG. 14 is a diagram illustrating an operation for a target cell tonotify candidate cells of a state change after a terminal performs HOaccording to an embodiment of the disclosure.

After a terminal 110 performs handover to a target cell 130 (steps 1410,1420, 1430, 1440, 1450), if the target cell 130 transmits HO complete toa serving cell 120 (step 1460), the serving cell 120 may manage an x2interface by transmitting serving cell relocation indication, includinga new target cell ID and a terminal ID, to all candidate cells 131.

<An Embodiment of an Operation for a Terminal to Switch to a CandidateCell if there is any One Candidate Cell without Shifting to an ExistingRe-Established Cell when an RLF Occurs Due to an MR Transaction Failureand Various Other Causes>

FIG. 15 is a diagram illustrating an operation for a terminal to switchto a candidate cell when an RLF occurs according to an embodiment of thedisclosure.

Even if a handover event for a given candidate cell has not beengenerated, an RLF may occur due to an MR procedure failure or otherfactors (steps 1510, 1520, 1530, 1540). When an RLF occurs, a terminal110 does not perform reestablishment, and may attempt handover to avalid candidate cell if the valid candidate cell to which the terminalcan switch is present (steps 1550, 1560, 1570, 1580).

A determination of a valid candidate cell to which a terminal can switchmay include a cell having the best cell quality in a current candidatecell list.

<An Embodiment of the Definition of a Handover Failure in UE-BasedHandover>

HO execution timer (execution time): a terminal 110 starts a timer fromthe moment when a UE-based HO execution event is initialized. This timermay be configured as an RRC dedicated message along with a UE HOcondition from a serving base station 120. The timer is stopped when theterminal 110 performs HO to a target cell and transmits an HO completemessage. If the timer expires, the terminal 110 considers it as an HOfailure and enters an RLF processing routine.

Total HO time: in another embodiment, when a terminal 110 receivestarget cell information, configuration information to be used in acorresponding cell, and a condition to be applied upon HO to thecorresponding cell through RRC dedicated signaling from a serving basestation 120, the serving base station 120 may provide a separate timervalue to the terminal 110 or provide a timer value to the terminal 110at any timing (system information or when a connection request is firstmade), so the terminal 110 may start the timer while receiving RRCdedicated signaling that forwards the condition. In this case, theterminal 110 may manage the separate timer for each given candidatetarget cell, and stops the timer when it transmits an HO completemessage because a UE HO condition for the corresponding cell is met andthus the terminal performs HO. If the timer expires, the terminal 110declares an RLF.

After the terminal 110 receives a measurement configuration for acandidate cell, when an RLF occurs before the terminal performs HO to atarget cell, the terminal 110 considers this as an HO failure and entersan RLF processing routine.

The terminal 110 receives a measurement configuration for a candidatecell. Thereafter, the terminal considers an RLF, occurring afterreceiving an HO command, as an HO failure and enters an RLF processingroutine.

The terminal 110 may perform UE HO although any one MR for adding acandidate cell is not triggered.

<Network Signaling Between Candidate Target Cells and a Serving Cell>

If a serving cell 120 notifies a candidate target cell 130 that theserving cell is a candidate target cell and transmits an HO requestmessage to the candidate target cell 130, the candidate target cell 130performs admission control. If a corresponding terminal 110 determinesto accept the serving cell 120 as a candidate target cell, the terminaldrives a timer while transmitting an HO request ack message to theserving cell 120. If the terminal 110 does not perform HO during thetimer, the candidate target cell 130 performs signaling that inquiresthe serving cell 120 about whether the terminal 110 is still valid.

A contention free random access (CFRA) timer: Separately, if thecandidate target cell 130 determines to admit a corresponding terminal110 and allocates a CFRA resource, the target cell 130 may manage aseparate timer during the time when the corresponding resource can beallocated. A value of this timer is also transmitted to the serving cell120. The serving cell 120 transmits the value of the timer to theterminal 110. Accordingly, the terminal 110 may also manage a timer forthe corresponding CFRA resource. The timer value is included ininformation, indicating that the target cell has been added as thecandidate target cell 130, and RRC dedicated signaling that provides aUE HO execution condition for the candidate target cell 130, and istransmitted from the serving base station 120 to the terminal 110. Theterminal 110 may start the timer simultaneously when it receives thesignaling. If the timer expires and a different timer (e.g., an HOexecution timer in a previous section or a total HO time) has not stillexpired, the terminal 110 does not declare an RLF, and may perform UE HObased on contention-based random access.

After a candidate target cell 130 transmits HO request ack to a servingcell 120 because the serving cell 120 notifies the candidate target cell130 that the serving cell is a candidate target cell, if the candidatetarget cell 130 can no longer reserve a resource for a UE-based HOterminal in a process of allocating its own resource to terminals 110out of its coverage or to a terminal 110 that performs network-basedhandover to the candidate target cell 130, the candidate target cell 130may transmit a signal to request network release to the serving cell120, and may immediately release a resource for the UE-based HO terminal110. Alternatively, the candidate target cell 130 may release theresource after receiving release ack from the serving cell 120.

In the case of a CU-DU, signaling may not be overhead.

If a divided CU-DU structure is present, each cell or base stationdescribed above may be mapped to the DU. The DU may include the protocolstack of PHY, MAC, and RLC, and the remaining L2/3 stacks may present inthe CU. In this case, when a target cell requires signaling thatnotifies candidate cells of a state change after a terminal 110 performsHO, or with respect to the signaling of an operation of processing acandidate cell and MR configuration used upon communication with aserving cell 120 after the terminal 110 performs handover, separate RRCsignaling is not necessary because RRC entities for respective cells arecrowded in a CU that manages multiple DUs, and may be substituted withRRC signaling through an interface within the CU.

FIG. 16 is a diagram illustrating the configuration of a terminalaccording to an embodiment of the disclosure.

Referring to FIG. 16 , a terminal 110 may include a transceiver 1610, acontroller 1620, and a storage 1630. In the disclosure, the controller1620 may be defined as a circuit or an application-specific integratedcircuit or at least one processor.

The transceiver 1610 may transmit and receive signals to and fromanother network entity.

The controller 1620 may control an overall operation of the terminal 110according to the embodiments proposed in the disclosure. For example,the controller 1620 may control a flow of signals between the blocks sothat the operations according to the flowchart are performed. Forexample, the controller 1620 may transmit a first message, includinginformation on a candidate target cell, to the serving base station 120,and may receive a second message, including information on a conditionfor conditional handover related to the candidate target cell 130, fromthe serving base station 120. If a measurement result for the candidatetarget cell 130 meets the condition for the conditional handover, thecontroller 1620 may perform handover to the candidate target cell 130.

The storage 1630 may store at least one of information transmitted andreceived through the transceiver 1610 and information generated throughthe controller 1620.

FIG. 17 is a diagram illustrating the configuration of a base stationaccording to an embodiment of the disclosure.

Referring to FIG. 17 , the base station may include a transceiver 1710,a controller 1720, and a storage 1730. In the disclosure, the controller1710 may be defined as a circuit or an application-specific integratedcircuit or at least one processor.

The transceiver 1710 may transmit and receive signals to and fromanother network entity.

The controller 1720 may control an overall operation of the base stationaccording to the embodiments proposed in the disclosure. For example,the controller 1720 may control a flow of signals between the blocks sothat the operations according to the flowchart are performed. Forexample, the controller 1720 may receive a first message, includinginformation on the candidate target cell 130, from the terminal 110, maytransmit a second message, including configuration request informationfor conditional handover, to the candidate target cell 130, may receivea third message, including configuration approval information for theconditional handover, from the candidate target cell 130, may configurea condition for the conditional handover on which the terminal 110perform handover to the candidate target cell 130, and may transmit afourth message, including information on the condition for theconditional handover related to the candidate target cell 130, to theterminal 110.

The storage 1730 may store at least one of information transmitted andreceived through the transceiver 1710 and information generated throughthe controller 1720.

<A Detailed Example of Signaling that a Serving Cell Designates a UE HOCondition for a Terminal>

In this embodiment, a detailed example in which the serving base station120 signals required information for UE HO independently of an operationfor the serving base station 120 to signal a separate M value to theterminal 110 is handled. Furthermore, in the description of allsubsequent embodiments, an NR-specific signaling field, such asreconfigurationWithSync instead of the mobilityControlInfo signalingfield, may be taken into consideration as a container that transmits aconditional HO command or a configuration in a required target cell uponHO and a condition for conditional HO (CHO).

In the case of all conditional HOs, the serving cell 120 may transmit atleast next information to the terminal 110. That is, the serving cell120 may transmit three types of information, that is, the cell ID of acandidate target cell, a reference measurement ID, and delta informationto be used when a measurement for conditional HO is performed bypartially changing a measurement corresponding to the ID, to theterminal 110. In this case, the reference measurement ID is one ofmeasurement IDs already configured in the terminal 110, and means ameasurement to which reference is made when a measurement operating tonewly perform conditional HO is configured in the terminal 110. In thiscase, the reference means that most of configuration informationconfigured in the measurement is reused. Only the delta information ofthe three types of information is newly substituted information.

The terminal 110 that has received the information performs a newmeasurement to which given delta info information has been appliedinstead of measurement configuration information of an indicatedmeasurement ID. In this case, the measurement is performed on the targetcell of the above-signaled cell ID.

If conditional HO is to be configured in multiple candidate targetcells, the serving base station 120 may signal, to the terminal 110, ameasurement ID, already maintained by the terminal 110 and being areference for delta info and update, for each candidate target cell ID.That is, the serving base station 120 may transmit a plurality of threetypes of information to the terminal 110. The reason for this is thatthere is an advantage in that signaling overhead can be reduced comparedto a case where all types of separate measurement configurationinformation are transmitted because most of the factors of a measurementconfiguration used by the terminal 110 are similar in most of types ofinformation in normal HO in addition to conditional HO. Furthermore, inthe existing event for normal HO, if an offset is 2 dB in A3,conditional HO needs to operate if received power of a serving cell issuddenly reduced, so an offset value of a greater value (e.g., 5 dB) maybe set in A3, that is, the same HO event, based on the original purposeof conditional HO. That is, if the serving base station 120 providesoffset=5 dB, a measurement ID in which the A3 event has been configured,and a target cell ID to the terminal 110 as described above, theterminal 110 uses all measurement configuration values previously usedin the A3 event, substitutes only the offset value with 5, and measuresthe target cell ID. If the measurement is performed as described aboveand the A3 event is met with respect to the offset value, the terminal110 does not transmit an MR to the serving base station 120 and directlyperformed HO to the target cell 130.

FIG. 18 is a diagram illustrating an operation method of conditional HOaccording to an embodiment of the disclosure.

When a terminal 110 transmits, to a serving base station 120,event-triggered results from a measurement configuration for pre-definednormal HO as an MR (step 1810), if the serving cell 120 transmits a CHOrequest to a candidate target cell 130 indicated in the corresponding MR(step 1820), the corresponding cell 130 identifies that HO is possiblebased on its own resource situation and configuration information of theUE, and transmits the ack of the request to the serving cell 120 againif possible (step 1830). The serving cell 120 makes a CHO condition forthe candidate target cell 130 (step 1840), invokes most of measurementconfiguration information configuring the condition, and provides areference measurement ID to be reused, delta info to be newly applied toa measurement in addition to the reuse, and a candidate target cell IDto the terminal 110 (step 1850). When the terminal 110 receives thisinformation, it performs a new measurement to which given delta info hasbeen applied in a measurement configuration corresponding to theindicated measurement ID. The subject of the new measurement is thecandidate target cell 130 given at step 1850 (step 1860). If an eventconfigured in a given measurement for CHO is established because a giventime elapses (step 1870), the terminal 110 performs HO to the candidatetarget cell 130 (steps 1880, 1885, 1890).

FIG. 19 is a diagram illustrating an operation method of conditional HOaccording to another embodiment of the disclosure.

In another embodiment, while a terminal 110 performs CHO (steps 1910,1920, 1925, 1930, 1940, 1950), if an event, meaning that a signal with acandidate target cell 130 is not good during the existing measurementseparately from a measurement for the CHO, has been configured and theevent is triggered, the terminal 110 notifies a serving cell of thecorresponding measurement results (step 1960). The serving cell 120requests the corresponding candidate target cell 130 to be released froma target cell for conditional HO because the corresponding candidatetarget cell 130 does not have an effective value (step 1970). If thecandidate target cell 130 provides the serving cell with a signal thataccepts the request (step 1975), the serving cell 120 transmits, to theterminal 110, a signal indicating that the terminal has to stop CHO withthe corresponding candidate target cell ID and release a correspondingresource (step 1980). The terminal 110 that has received the signalstops a measurement operation for CHO for the corresponding candidatetarget cell 130 and releases the existing resource configuration for acandidate target cell (step 1990).

FIG. 20 is a diagram illustrating an operation method of conditional HOaccording to yet another embodiment of the disclosure.

In another embodiment, while the terminal 110 performs CHO (steps 2010,2020, 2025, 2030, 2040, 2050), if a candidate target cell 130 givespriority to a different terminal 110 not based on results measured bythe terminal 110 and performs resource allocation, terminal allocationfor CHO may be released. If the candidate target cell 130 notifies aserving cell 120 (step 2070) of CHO resource release request contentsattributable to the priority of the different terminal (step 2060), theserving cell 120 transmits a corresponding candidate target cell ID andrelease indication to the terminal 110 (step 2080). In response thereto,the terminal 110 stops a measurement performed toward the correspondingcandidate target cell 130, and releases resource configurationinformation to be used in the candidate target cell 130 (step 2090).

FIG. 21 is a diagram illustrating an operation method of conditional HOaccording to yet another embodiment of the disclosure.

In another embodiment, when a serving cell 120 transmits suchinformation on the candidate target cell 130 to the terminal 110,quadruplet information may be given to the terminal 110 like (a newmeasurement ID for CHO, a reference measurement ID, delta info, and atarget cell ID). In this case, a portion newly included compared to theprevious example is the “new measurement ID for CHO.” If the terminal110 receives the ID, the terminal 110 makes a new measurement to be usedfor CHO based on the existing triplet (reference measurement ID, deltainfo, and target cell ID) information, but newly allocates an ID to themeasurement as a value of “new measurement ID for CHO.” In a method ofexplicitly allocating an ID to a measurement for CHO, the ID may be usedfor the serving base station 120 to modify or release a measurementcompared to a method of making and using a measurement for CHO withoutan ID. Furthermore, if a given candidate target cell has multiplemeasurement configurations, it is difficult to refer to a given one ofthe multiple measurement configuration and to modify or release thegiven measurement configuration, in preparation for this case, it isnecessary to allocate a separate measurement ID for CHO to eachmeasurement configuration for CHO so that the serving cell 120 cansubsequently manage the measurement for CHO.

In FIG. 21 , as in FIGS. 18 to 20 , when a terminal 110 transmits, to aserving base station 120, a report indicating that a pre-configuredevent is triggered and a candidate target cell 130 may become acandidate for CHO through a normal measurement (step 2110), the servingbase station 120 makes an HO request (step 2120) from the correspondingcandidate target cell 130. The candidate target cell 130 transmitsconfiguration information to be used in the candidate target cell 130 tothe serving cell 120 through ACK (step 2125). The serving base station120 transmits a condition for CHO to the terminal 110 along with theconfiguration information. In this case, the serving base station 120transmits a plurality of reference measurement IDs and delta info to thesingle candidate target cell 130 (steps 2130, 2140). When theinformation is received, the terminal 110 configures a different type ofmeasurement for CHO and performs a plurality of measurements on thecandidate target cell 130 (step 2150). If a meas ID1 for CHO is to beremoved at any timing (step 2160), the serving base station 120 maysignal that the terminal 110 should release the corresponding meas ID1for CHO (step 2170). When the signal is received, the terminal 110performs only a measurement for a meas ID2 configured in the candidatetarget cell 130 and stops the measurement for the remaining meas ID1(step 2180). In this case, if a measurement for CHO is to be removed fora given reason, the corresponding reason may be a case indicative of anevent in which information indicating that the radio state of thecorresponding candidate target cell 130 is not suitable for the meas ID1is received from the UE 110 through an MR or a resource situation ischanged so that the candidate target cell 130 does not guarantee givenlink quality and the meas ID1 for CHO guarantees such link quality.

In another embodiment, the terminal 110 is not notified of a measurementfor CHO through a reference measurement ID and delta info as in theexamples, but is notified of all types of required information necessaryfor CHO through an independent measurement configuration. The terminal110 that has received the information may configure a new measurementfor CHO without the existing reference measurement without any change.In a more detailed example related to this, independent measurementconfiguration information may be divided into absolute measurementobject information and report configuration information. All of types ofindependent information of the serving base station 120 are transmittedto the terminal 110. Even in this case, a new measurement ID for CHO maybe newly allocated explicitly or may not be allocated. If a measurementID has been explicitly allocated, when the serving base station 120subsequently tries to release a corresponding measurement forconditional HO, it may denote a meas ID and release the correspondingmeasurement. If a measurement ID is not explicitly allocated, theserving base station 120 may release (or remove) a correspondingmeasurement by signaling a candidate target cell ID to the terminal 110.If an ID is explicitly allocated, the existing reference meas ID and anew CHO meas ID may be separately managed.

In all the above-described embodiments, delta info may become thefollowing information or may become a combination of correspondinginformation.

(Information related to a report configuration) this may be at least oneof pieces of information, including an event type (e.g., this may meanan event present on a reportConfig IE, such as A1—A6, B1, A2, C1, or C2used in LTE), threshold and power offset information necessary for thecorresponding event type, a hysteresis value, measurement quantity,information indicating whether to include beam measurement results,information indicating whether to trigger ReportOnLeave, or a time totrigger value, information indicating whether a report is a periodicreport, measurement trigger quantity information, report quantityinformation, or an NR RS type. Furthermore, information related to ameasurement object may be given.

Furthermore, (information related to a measurement object) this may be areceived power offset value for a given measurement frequency, a cell IDto be measured, cell information to which alternative TTT will beapplied, information indicating whether to apply t312, a received powerindividual offset value for a given designated cell, subframeinformation for performing a measurement, SSB configuration information(e.g., time window and frequency location information of an SSB, SMTCinformation, or subcarrier spacing information) or CSI-RS configurationinformation (a cell ID, a bandwidth, CSI-RS time/frequency locationinformation, density, a CSI-RS index list, slot config, or an associatedSSB index) of a corresponding cell, a beam ID to be measured in acorresponding cell or the number of beams necessary when a beam ID iscalculated and received power threshold value-related information, L3filter value information for conditional HO, a minimum threshold valueof received power of a beam to be used in a measurement, or the numberof SSBs or CSI-RSs that will take an average value.

In another embodiment, if the serving base station 120 configures an Mvalue in the terminal 110 with respect to a given event (140 in FIG. 1), the terminal 110 may sequence target cells in order of the best linkperformance, among cells meeting the event (140 in FIG. 1 ), always attiming in which an event is triggered unlike a case where target cellsare put into a candidate cell set in a time sequence in which acorresponding event is triggered, and may maintain upper M target cellsas a candidate target cell. To this end, the terminal 110 may alwayssequence the target cells in order of link performance measured for eachtarget cell at timing in which a configured event is triggered. When theterminal 110 selects M best cells from the cells meeting the event (140in FIG. 1 ) based on the sequence, if the selected M best cells aredifferent from M candidate target cells already reported to the servingbase station 120, the terminal 110 may transmit, to the serving basestation 120, a signal that changes the selected M best cells. The signalmay include information on a cell to be excluded from the existingcandidate target cell and a cell that needs to be added because the cellis not present in the existing candidate target cell. When the servingbase station 120 receives the information, it performs a candidate cellremoval procedure on the cell to be removed and performs a candidatecell addition procedure, including handover preparation and ack, on thecell to be added.

As described above, in addition to measured cell link quality, a degreethat a cell is busy or measured received intensity information of only agiven reference signal may be considered as a metric for the selectionof M candidate target cells of the terminal 110. For example, if a casewhere received intensity of a given RS is a given value or more isdiscovered for a given period by a given number of times, this mayindicate that a cell is busy. The terminal 110 may store M best cellsbased on the metric and notify the serving base station 120 of thestored M best cells.

The technology proposed in this patent may also be applied to othercellular systems. For example, in the case of an unlicensed frequencyband system, all terminals 110 may perform listen-before-talk (LBT) andperform transmission only when a condition is met. Each cell can alsoperform transmission only when the condition is met. Accordingly, as inthe existing cellular system, delay attributable to LBT may occur incontrol plane signaling, such as a measurement report and a handovercommand. In this case, a terminal may enter an RLF because it does notperform handover at a proper time. In order to prepare such a case, ifthe serving base station 120 provides the terminal 110 with an HOexecution condition and corresponding target cell information in advanceand thus the condition is met, the terminal 110 may perform handoverwithout separate signaling along with the serving base station 120.

Furthermore, in the case of V2X or an aerial vehicle, if coveragematching with the moving speed of the terminal 110 and a base station isnot properly performed, control plane signaling may be unstable. Inorder to solve this problem, the serving base station 120 may notify theterminal 110 of a condition for UE-based HO and target cell informationin advance.

The embodiments of the disclosure disclosed in the specification anddrawings have suggested given examples in order to easily describe thetechnical contents of the disclosure and to help understanding of thedisclosure, and are not intended to limit the scope of the disclosure.That is, it is evident to those skilled in the art to which thedisclosure pertains that other modified examples based on technicalspirit of the disclosure may be practiced. Furthermore, the embodimentsmay be combined and operated, if necessary.

What is claimed is:
 1. A method performed by a terminal in a wirelesscommunication system, the method comprising: receiving, from a servingbase station of a source cell, a first message for conditional handoverconfiguration, the first message comprising first information on atleast one candidate target cell and second information on at least oneexecution condition for a conditional handover; in case that a radiolink failure (RLF) for the source cell occurs, staying in a radioresource control (RRC) connected state and identifying a valid candidatecell; and in case that the valid candidate cell is one of the at leastone candidate target cell, transmitting, to a target base station of thevalid candidate cell, a second message for the conditional handoverbased on the first information, wherein an RRC re-establishmentprocedure is not performed.
 2. The method of claim 1, further comprisingin case that the valid candidate cell is not any of the at least onecandidate target cell, performing the RRC re-establishment procedureafter the RLF for the source cell occurs.
 3. The method of claim 1,wherein the transmitting of the second message for the conditionalhandover further comprises applying a corresponding configuration forthe valid candidate cell included in the first information.
 4. Themethod of claim 1, wherein the first information on the at least onecandidate target cell includes identifier (ID) information of the atleast one candidate target cell.
 5. The method of claim 1, wherein thesecond information on the at least one execution condition for theconditional handover includes measurement ID information.
 6. A terminalin a wireless communication system, the terminal comprising: atransceiver; and a controller configured to: receive, from a servingbase station of a source cell via the transceiver, a first message forconditional handover configuration, the first message comprising firstinformation on at least one candidate target cell and second informationon at least one execution condition for a conditional handover, in casethat a radio link failure (RLF) for the source cell occurs, stay in aradio resource control (RRC) connected state and identify a validcandidate cell, and in case that the valid candidate cell is one of theat least one candidate target cell, transmit, to a target base stationof the valid candidate cell via the transceiver, a second message forthe conditional handover based on the first information, wherein an RRCre-establishment procedure is not performed.
 7. The terminal of claim 6,wherein the controller is further configured to in case that the validcandidate cell is not any of the at least one candidate target cell,perform the RRC re-establishment procedure after the RLF for the sourcecell occurs.
 8. The terminal of claim 6, wherein, to transmit the secondmessage for the conditional handover, the controller is furtherconfigured to apply a corresponding configuration for the validcandidate cell included in the first information.
 9. The terminal ofclaim 6, wherein the first information on the at least one candidatetarget cell includes identifier (ID) information of the at least onecandidate target cell.
 10. The terminal of claim 6, wherein the secondinformation on the at least one execution condition for the conditionalhandover includes measurement ID information.